Measured and modeled hand forces and resulting forces at the low back during the pull phase of a lifting task

The purpose of this investigation was to compare measured and modeled hand forces applied to the load during two-handed floor-to-knuckle Lifting tasks in the sagittal plane. The goal of this objective was to determine if dynamic biomechanical models should be supplemented with additional information to accurately model hand forces.

Hand force was measured with a strain gage instrumented apparatus which simulated a box-type container with handles. The measured hand forces were synchronized with video joint displacement data and load liftoff. Five male subjects performed lifting tasks for two experiments to determine the effects of speed of lifting motion, frequency of lift, percentage of maximum acceptable weight and weight of load on peak hand forces and resulting peak forces at the low-back. A dynamic biomechanical model was used to calculate hand forces and low-back forces.

Measured and modeled hand forces were comparable for the middle (carry) portion of the lift, but deviated substantially at the beginning (pull) and ending (placement) phases of the lift. The measured hand forces exhibited a steep spike during the pull phase of the lift which was not evident in the modeled hand forces. The peak magnitudes of the measured hand forces were significantly greater than the modeled values for all lifting tasks. The occurrence of peak measured hand forces was distributed about the liftoff, indicating that peak hand force occasionally occurred just before liftoff.

The peak compression and shear forces at the low-back calculated with input of measured hand forces were greater than peak modeled low-back forces for the normal, comfortable speed of lift, and significantly greater for the fast speed of lift. The steep peak at the pull phase in compression and shear forces at the low-back plotted over the duration of the lifting tasks was also evident for most lifting tasks.

The results of this investigation indicate that the dynamic biomechanical model should be supplemented with a static model of the pull phase of the lift to more accurately represent the peaking of actual hand forces and the resulting compression and shear forces at the lowback.